Fans or blowers are used along with ductwork to circulate air through a room or area of a building. The blower typically draws air from within the room through a return air duct and then forces the air back into the room through a supply air duct. To heat or cool the air, the blower may also force or draw the air across a heat exchanger.
To help prevent dust from accumulating on the heat exchanger, blower, and ductwork, often a conventional filter is installed at the downstream end of the return air duct. Finer, less porous filters are used where dust removal is more critical, such as in so called clean rooms or in buildings having occupants with dust-related allergies. Unfortunately, fine filters usually create a higher pressure drop that reduces the amount of airflow. To minimize the pressure drop, an effective cross-sectional area of the filter can be increased in various ways, such as by adding pleats to the filter, installing the filter at an angle relative to the duct, or by forming the filter as an elongated bag that extends lengthwise into an air duct.
Some examples of filters that are elongated along the direction of airflow are disclosed in U.S. Pat. Nos. 2,853,154; 3,151,962; 3,195,296; 3,204,391; 3,204,392; 3,396,517; and 3,538,686. When mounting such filters within a return air duct, upstream of the blower, a significant distance is needed between the blower and where the filter attaches to the duct, simply due to the length of the filter. In many cases, this can be difficult or impossible to do, because of bends or elbows in the ductwork. Also, much of the ductwork is usually inaccessible, as it is often installed within the walls of the building or between the floor and ceiling. Filters in a return air duct are therefore typically installed immediately adjacent the blower, which may prohibit the use of an elongated filter or at least significantly limit its length.
On the other hand, if an elongated air filter were installed in the supply air duct, the filter would do little in preventing dust from accumulating on the blower and the heat exchanger, because dust often originates in the room. With a filter installed in the supply air duct, dust from the room could pass across the blower and heat exchanger before ever reaching the filter.
Moreover, if elongated filters of current designs were installed within a generally cylindrical duct having a pliable fabric wall, the non-conical shape of the filter may cause the fabric of the duct to flutter, due to uneven patterns of airflow velocity. If the cross-sectional area of airflow between the exterior of an elongated filter and the interior of the cylindrical fabric duct is not circumferentially uniform, as could be the case with a flat-sided filter within a cylindrical duct, localized areas of higher velocity may exist. Also, abrupt changes in velocity along the length of a fabric duct may also cause the fabric to flutter.